The present invention relates to a hydraulic valve having a kinked characteristic curve, more particularly, for a hydraulic consumer of a vehicle with a first and second valve body which are arranged axially displaceable in one bore and each have at least one control surface which can be biased by a control pressure fluid. The invention also relates to a motor vehicle having a hydraulic pressurized medium unit including a hydraulic consumer and at least one pressurised medium pump along with a hydraulic valve.
A hydraulic valve having a kinked characteristic curve for a hydraulic consumer of a vehicle is used for example when the hydraulic consumer has to meet performance demands which can be overall quite different. One such hydraulic consumer can be for example a multi-plate clutch operated by pressurised medium and having to cover in the vehicle a wide torque range since the vehicle in normal driving mode is operated as a rule with a low torque provided by the engine whilst the clutch however has to be capable of functioning even when there is a high torque arising on the engine side.
The hydraulic valve thereby serves to control the hydraulic consumer with a pressurised fluid as work fluid. During transfer of the engine torque to the drive train through a hydraulically operated clutch it is necessary therefore to meet two conditions, namely on the one side the maximum torque has to be transferred, which means that a certain maximum pressure has to be reliably produced, and on the other hand when starting off, whilst the clutch is slipped, the transferred torque has to be capable of the most precise possible finely measured adjustment so that the greatest possible comfort can be obtained.
The matter of comfort can be achieved with a relatively flat valve characteristic curve because, if disturbing factors arise, such as for example the appearance of hysteresis, changes in temperature which alter the viscosity of the preseurised fluid, dirt contamination or changes in friction, in the case of a flat valve characteristic curve only alight changes occur in the work pressure which are scarcely noticeable. However with such a flat valve characteristic curve the initial requirement of rapidly reaching a certain maximum pressure cannot be fulfilled.
A kinked valve characteristic curve can now be used to meet both requirements. This has a small rise in the case of low fluid flows and from a certain kink point has a large rise.
DE 197 21 036 A1 already discloses a hydraulic valve having a kinked characteristic curve and a vehicle having a pressurised medium unit with a hydraulic valve of this kind. Although the hydraulic valve known here has proved satisfactory, in practice it has as a result of its design with a seat valve the drawback that the seat valve has a constant through-flow after exceeding the kink point and thus a leak occurs and as a result of the complex flow conditions instability of the kink line can arise in the seat valve. It has also been seen that in the event of temperature changes the characteristic curve can undergo displacement and as a result of manufacturing tolerances it is necessary to adjust the valve during assembly.
The object of the present invention is therefore, in order to overcome the drawbacks described, to provide a hydraulic valve having a kinked characteristic curve which avoids the said adjustment expense during assembly and which is substantially insensitive to displacements of the characteristic curves. Furthermore a motor vehicle having a hydraulic pressurised medium unit and a hydraulic consumer is to be provided which can be supplied with a work fluid and has a predeterminable reliable operating behaviour.
The invention provides a hydraulic valve having a kinked characteristic curve for a hydraulic consumer of a motor vehicle, with a first and a second valve body which are each mounted axially displaceable in a bore and each have at least one control face which can be biased by a control pressure fluid wherein the one valve body is, provided with two pressure return faces which can be biased by a presourised fluid and which counteract the pressure biasing of the control surface, and wherein the other valve body controls the biasing of at least one of the pressure return faces.
Thus a hydraulic valve is provided having two pressure return faces which can be biased by a pressurised fluid, namely so that the biasing of the pressure return faces counteracts the biasing of the control face of the valve body wherein the biasing of one of the pressure return faces is controlled by the other of the two valve bodies. In the case of a relatively low control pressure the two pressure return faces are biased, whereupon a flat characteristic curve is produced which can change from a predetermined control pressure into a steeper characteristic curve from which at least one pressure return surface is biased with a constant or a decreasing return pressure.
According to a further development of the invention the two valve bodies are formed as a valve piston which in the region of the piston faces with the exception of slight leakage flows have no noticeable through-flows so that temperature-conditioned changes in the viscosity of the pressurised fluid do not lead to any displacements of the characteristic curve of the hydraulic valve.
The two pressure return faces on one of the valve pistons can be formed different size or however can also have the same sized active surface areas which can be biased by the pressurised fluid.
In a continuation of the invention it is proposed that at is least one of the two pressure return faces on the valve piston is biased by a pressurised fluid with variable pressure level. The pressurised fluid for biasing the at least one pressure return surface can be a work fluid supplied by a pump to the hydraulic valve under pressure for the hydraulic consumer so that through a connecting pipe the pump supplies the hydraulic valve with a volume flow of work fluid which is to be supplied to the hydraulic consumer through the hydraulic valve. Through a change in the cross-sectional area of an inlet supply of the hydraulic valve which takes place through an axial displacement of the valve piston in the bore of the hydraulic valve, work fluid is supplied to the hydraulic consumer which is also guided at the same time to a pressure return face of the valve piston, namely so that through the pressure of the returned work fluid to the pressure return face of the valve piston a force is produced which acts opposite a pilot or precontrol force which is produced by the control pressure fluid at the control surface of the valve piston.
At the same time the other pressure return face on the valve piston is biased with a pressurised force which acts opposite the force action through the control pressure biasing of the control surface of the valve piston, which from the proposed kink point of the valve characteristic curve is biased with a pressurised fluid of a substantially unchanged pressure level. This means that with the increase in the control pressure, biasing of the valve piston first the two pressure return faces on the valve piston are biased through for example the work fluid provided for the hydraulic consumer and that then the fluid supply to the second pressure return surface from the proposed kink point of the valve characteristic curve is interrupted or restricted through an axial displacement of the second valve piston so that from this time point the second pressure return surface is biased with a substantially unchanged, that is substantially stationary, pressure level, so that the force components stemming from the second pressure return surface and counteracting the force components produced by the control pressure biasing of the control surface is changed no more from the predetermined kink point of the valve characteristic curve. Further biasing of the control surface of the valve piston having the two pressure return surfaces through the control pressure fluid therefore leads to a further axial displacement of this valve piston in the bore of the hydraulic valve and thus to a rising volume flow of the work fluid supplied by the pressurised medium pump so that the pressure level of the work fluid supplied to the hydraulic consumer rises.
According to a further development of the invention it is possible that the other pressure return surface from the predetermined kink point of the valve characteristic curve is indeed furthermore biased with a pressurised fluid which has a changing pressure level, but this pressure level is however decreasing. To this end the valve piston which is used to control the biasing of the pressure return faces, can act as a regulating valve so that the volume flow supplied to the second pressure return face is reduced through the pressure regulating valve with rising pilot pressure at the valve piston of the regulating valve so that the force components which stem from the second pressure return face and which counteract the force components of the control pressure biasing of the valve piston having the two pressure return faces decreases to an increasing amount. Since at any considered moment the force component acting through the control pressure biasing of the control face on the valve piston having the two pressure return faces has to be balanced with the oppositely acting force components through the two pressure return faces, this design leads to a rise in the force component which is produced by the constantly biased pressure return surface.
A particularly space-saving design of the hydraulic valve can be produced if one of the valve pistons, preferably the valve piston having the two pressure return surfaces, has an internal bore in which the other valve piston is housed axially displaceable. The valve piston having the two pressure return surfaces which influences the supply of the hydraulic consumer with work pressure acts as a pressure regulating valve whilst the valve body controlling the biasing of the pressure return surfaces can act as a pressure reducing valve or as a pressure regulating valve or also as a route valve.
The control pressure fluid which biases the control surface of the valve piston having the two pressure return surfaces is a pilot pressure fluid supplied to the hydraulic valve through a for example independent pump. The control pressure fluid supplied to the corresponding control surface of the second valve piston can likewise be an independent pilot pressure fluid for example a pilot pressure fluid likewise supplied to the first valve piston having the pressure return faces, or in the case of the hydraulic valve designed with two interfitting displaceable valve pistons can be the work fluid supplied to the hydraulic consumer.
According to the invention a motor vehicle is also provided having a hydraulic pressurized medium unit, at least one hydraulic consumer and at least one pressurized medium pump wherein the motor vehicle has at least one hydraulic valve, such as already described above, which is provided for supplying the hydraulic consumer with a pressurized fluid as work fluid, wherein the pressure of the work fluid is substantially proportional to the control pressure of the control pressure fluid and from a predetermined control pressure (Pkink) the proportional ratio is increased relative to the region below the predetermined control pressure. This means that a work fluid is supplied to the hydraulic consumer of the motor vehicle through the hydraulic valve so that in the first operating range the pressure of the work fluid runs substantially proportional to the pressure of the control pressure fluid whilst the proportional ratio between the pressure of the work fluid and the pressure of the control pressure fluid is increased. In the first region therefore a change in the control pressure leads to a comparatively slight change in the pressure of the work fluid which is supplied to the hydraulic consumer so that the pressure of the work fluid in this first region can be changed with fine sensitivity through a change in the control pressure and a change in the control pressure after a change in the proportional ratio leads to a clearly marked change in the pressure of the work fluid. A change in the input characteristic value control pressure therefore results in the first region only in a slight change in the starting characteristic value pressure of the work fluid, whilst after the change in the proportional ratio a change in the input characteristic value control pressure leads to a clearly more marked change in the output characteristic value pressure of the work fluid.
It is thereby possible that the proportional ratio changes substantially suddenly but it can also be changed continuously.
The hydraulic consumer of the motor vehicle can thereby have a wet-running multi-plate clutch and/or a hydrodynamic converter which can also have a converter lock-up clutch, and/or a transmission wherein according to a preferred embodiment the transmission is a continuously variable transmission which has two pairs of cone pulleys whose piston/cylinder units are biased with the work fluid. Biasing the piston/cylinder units with the work fluid supplied through the hydraulic valve can thereby be both dependent on transmission and dependent on the torque which is to be transferred from the continuously variable transmission to the drive train of the motor vehicle.